LITTLE ROCK (AP) — A civil liberties group asked a judge to block Arkansas from enforcing its new voter ID law while he considers a lawsuit challenging the requirement's constitutionality.

The American Civil Liberties Union of Arkansas filed a motion for a preliminary injunction against the law, which requires voters at the polls to show photo identification before casting a ballot. The group and the Arkansas Public Law Center sued the state over the law last week, saying it violates Arkansas' constitution.

The groups sued on behalf of four voters who they said would be harmed by the law.

The May 20 primary will be the first statewide test of the new law, which was approved by the Legislature last year. Early voting for the primary begins May 5. The groups said there's not enough time to weigh their challenge before voters begin casting ballots.

"As early voting starts within the month, there is not enough time to accomplish this considering what time that would remain after preparing and trying a lawsuit to conclusion," the groups said in a brief filed in Pulaski County court.

Democratic Gov. Mike Beebe vetoed the voter ID measure, but lawmakers overrode his objection with simple majority votes in the House and Senate. The ACLU lawsuit names Secretary of State Mark Martin and the state Board of Election Commissioners as defendants.